In the fanfiction, it could be that the overall situation forces Harry to trust Snape. For instance, all other allies seem to be untrustworthy or have been compromised. Snape, on the other hand, presents himself as the only one with a clear plan to defeat the main antagonist. Harry, having no other options, decides to trust Snape.
Perhaps in this fanfiction, Snape shows a side of himself that is completely different from what Harry has known. He might be more open and honest with Harry, sharing secrets that make Harry believe he can be trusted. Like Snape might disclose some long - held information about Harry's parents that only he knew, which makes Harry think Snape has been on his side all along.
Perhaps in this fanfiction, Snape has proven himself to Harry in some extraordinary way. Maybe he has saved Harry's life multiple times or has shown his true loyalties that were hidden in the original story. It could also be that Snape has some special connection with Harry that makes Harry trust him unconditionally.
One possible reason could be that Snape shows a different side to Harry in the fanfiction. Maybe he protects Harry secretly from some hidden threats that others are unaware of, which makes Harry start to trust him.
A plotline could be that Snape is framed for a crime he didn't commit, and Harry, who trusts him, is the only one who believes in his innocence. Harry then sets out to prove Snape's innocence, facing many obstacles and enemies in the process. This would be a great twist on the typical relationship between them in the original series.
As the story progresses, they might start to work together more closely. Snape could be teaching Harry some advanced magic that no one else knows, and through this process, they build a bond. Harry begins to understand Snape's past better and realizes that they have more in common than he thought, like their love for Lily Potter in different ways.
One reason could be that Harry sees Snape's Patronus, which is the same as his mother's. This could be a strong indication of Snape's true feelings and make Harry trust him.
One reason could be that in this fanfiction, Snape has some hidden kindness towards Harry that he doesn't show in the original story. Maybe he secretly protects Harry from other threats, which makes Harry feel safe around him.
In some fanfictions, yes. Writers often explore the complex relationship between Harry and Snape. They might create scenarios where Harry discovers Snape's true loyalties and motives earlier than in the original books, leading to a new level of trust.
In this fanfiction, their relationship could develop in a rather unexpected way. Maybe Snape initially approaches Harry for his own reasons, but as time goes on, a real bond forms. Harry, who has always been in need of someone to trust, finds in Snape a kind of stability. Snape, on the other hand, might change his attitude towards Harry completely. He might stop being so cold and distant and instead become a warm and reliable figure for Harry. This transformation in their relationship is likely to be a central part of the fanfiction's plot.
One possible reason could be Snape's lingering resentment towards James Potter, Harry's father. Snape might see Harry as a reminder of the bullying he endured from James during their school days. So, he takes out his anger on Harry.
It could be that Snape teams up with Harry to solve a mystery that is crucial to the wizarding world. During their cooperation, Snape shows his loyalty and intelligence. He may sacrifice some of his own interests for the greater good. Harry witnesses all these and realizes that Snape is not the person he thought he was, thus learning to trust him.